The present invention generally relates to medical devices for stabilizing the vertebral motion segment. More particularly, the present invention relates to a composite spinal intervertebral body cage for distraction and fusion.
Certain known spine cages or implants are characterized by a body comprising a hydroxyapetite coated surface provided on the exterior surface for contact with adjacent vertebral segments or endplates. A cage of this type may be inserted posteriorly through the neuroforamen of the distracted spine after a surgeon removes disc, bone, and ligament material to create a pathway.
Such existing devices for interbody stabilization have important and significant limitations. Current devices for interbody stabilization include static spacers composed of titanium, PEEK, and high performance thermoplastic polymer produced by VICTREX, (Victrex USA Inc, 3A Caledon Court; Greenville, S.C. 29615), carbon fiber, or resorbable polymers.
One problem with conventional devices for interbody stabilization made of PEEK, other high performance thermoplastics or resorbable polymers is the relative weakness and/or brittleness of these materials compared to the forces required to insert the device between bones of the spinal column. A review of the Food and Drug Administration's Medical Device Reporting (MDR) database for intervertebral body cages show that the greatest reported failure rate, at 36% of all reports, is for breakage of the cage during insertion. Therefore there is a need for intervertebral body cages made from materials that can withstand the insertion forces without breaking.
The failure point for most cages experiencing breakage during insertion is the point of attachment between the intervertebral body cage and the inserter attached to the cage which is used to place the cage between the vertebrae. There are many means know to those skilled in the art for attaching a spinal fusion cage to an insertion instrument, including, but not limited to a threaded hole and threaded screw, an impression or indentation and hooks or projections, and a supporting surface and a clamping mechanism. In all cases, the attaching means must not only secure the spinal fusion cage to the inserter and then release the cage once it is properly located in the intervertebral space, but the attaching means must also provide a secure attachment during the insertion step when significant forces may be required to advance the cage between vertebral bodies that have come in contact or near contact around a “collapsed” disc space.
Impact loads of greater than 50 pounds force have been measured during the insertion of intervertebral spinal cages between vertebrae. Even more challenging can be the rotational moments placed on the implant as it is forced into a rigidly defined space as more than 90 inch-pounds of torque have been recorded during insertion. Therefore there is a need for intervertebral body cages with robust insertion attachment which can withstand the insertion forces without separation.